Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of the target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target race-way.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known dispositions in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to each other by use of soldering, brazing, diffusion bonding, clamping, epoxy cements, etc.
To a certain extent, soft solders can accommodate stresses exerted on the target/backing plate assembly that occur upon cooling. These stresses can be considerable in light of the significant differences in thermal expansion coefficients that may exist between the target and backing plate metals. However, the relatively low joining temperatures associated with the "soft" solders reduce the temperature range over which the target can be operated during sputtering.
In some cases, in order to overcome the problem of joining one or more non-wettable materials by soldering, precoating with a metal is used to enhance solderability. These coatings may be applied by electroplating, sputtering or other convenient methods.
Another method which is applicable and used to some extent in target joining is that of explosive bonding or welding. By this technique, bonds are produced that combine solid state bonding and a mechanical interlocking as a result of the surface irregularities produced in the form of "jetting." The bonds are strong and reliable. The disruption of the initial mating surfaces during the dynamic bonding pulse negates the need for extreme surface cleanliness or preparation.
Diffusion bonding is an applicable method of bonding but has had only limited use in the bonding of sputtering target components. The bond is produced by pressing the material surfaces into intimate contact while applying heat to induce metallurgical joining and diffusion to varying extent across the bond interface. Bonding aids, metal combinations which are more readily joined, are sometimes applied to one or both of the surfaces to be bonded. Such coatings may be applied by electroplating, electroless plating, sputtering, vapor deposition or other usable technique for depositing an adherent metallic film. It is also possible to incorporate a metallic foil between bonding members which foil has the ability to be more easily bonded to either of the materials to be joined. The surfaces to be joined are prepared by chemical or other means to remove oxides or their films which interfere with bonding.
Solder bonds of materials with widely differing thermal expansion rates are susceptible to shear failure initiating at the extreme edges of the bond interface when the solder is too weak for the application. The result commonly experienced is debonding during service. The need for intermediate coatings applied to materials that are difficult to wet and solder presents two problems, 1) adherence reliability of the applied coating and 2) substantial added cost of applying the coating. The higher melting temperature solders used for high power applications are stronger but are less forgiving of the stresses developed in the material system. Targets of large size present greater stress problems as well as greater difficulty of producing sound bonds across the entire bond surface. As sputtering target sizes and power requirements increase, the soft solders become less applicable for joining of the material systems involved.
Explosive bonding is a comparatively costly bonding method. Such bonding requires that the materials to be joined are provided in an oversize condition, allowing for predictable damage at the periphery of the target assembly, thereby adding material cost. Each size of component assembly and combination of materials requires development of the conditions for achieving acceptable products. Although the bonds offer good strength the bond interfaces are variable in physical character. This method is not applicable to a material system which has one component which is brittle or has limited ductility.
Diffusion bonds require extreme care in preparation and in maintaining surface cleanliness prior to and during the bonding operation to ensure reliable bond qualities. The diffusion bond interfaces being planar are subject to stressing in simple shear which commonly leads to peeling away at the ends of the bond area. The formation of brittle metallics at the bond interface, which increase in thickness with the associated long times of heat exposure, add to the potential of bond shear failure.
Accordingly, it is an object of the invention to provide a convenient, inexpensive method for bonding target and backing plate materials that will be capable of withstanding thermal expansion and contraction stresses exerted thereon during and after sputtering.